Generally, there are several kinds of systems for measuring pressure and/or temperature of a tire. Among them, a system using a SAW sensor measures pressure and/or temperature of a tire and sends information on the measured pressure and/or temperature through wireless communication. The system using a SAW sensor has an advantage that a SAW sensor does not need an inner power supply.
A typical system for measuring pressure and/or temperature of a tire using a SAW sensor includes an antenna and a SAW sensor. The antenna receives micro-waves from an external device and sends information on pressure and/or temperature of the tire back to the external device. The SAW sensor generally includes a transducer and a plurality of reflectors, typically three, arranged in a straight line. The transducer receives micro-waves from the antenna and converts them into a SAW (Surface Acoustic Wave). The converted SAW travels via the reflectors and finally returns to the transducer. The returned SAW is converted back to micro-waves, and the converted micro-waves are transmitted to an external device through the antenna.
Tire temperature can be measured because the travel speed of the SAW varies according to the temperature of surrounding air. In addition, if an impedance is connected to one of the reflectors, the quantity of the reflected SAWs is changed in response to the impedance. Therefore, by connecting a capacitive sensor with impedance that varies according to pressure to a second reflector, the quantity of SAWs reflected will be varied. The pressure can thus be determined by comparing the quantity of SAWs reflected as between different reflectors.
However, there are certain disadvantages to such systems. A reflected SAW is transmitted through all reflectors sequentially due to the straight line arrangement, and therefore must pass the first and last reflector twice. Thus, the transmissivity of the reflectors must be greater than reflectivity thereof in order for the SAW transmitting through a first reflector to return to the first reflector via a last reflector. Therefore, reflectivity of each of the reflectors must be designed to be less than transmissivity thereof.
Such a system has a problem in that a micro-wave output from the transducer becomes smaller than a micro-wave input thereto. In addition, since the system uses wireless communication in order for the saw sensor to communicate with an external device, the system generally uses a higher frequency than frequencies typically used for a hard-wired communications.
Under such high-frequencies, a problem can occur in that the gap between electrodes of the transducer and reflectors becomes smaller. As an example, in order to produce a tranducer with a frequency of 430 MHz using a board made of LiNbO3 (LN; Lithium Niobate), the gap between electrodes of the transducer must be about 2 μm. Accordingly, in order to produce an transducer having frequencies higher than 430 MHz, the gap between electrodes must be further decreased, so that it becomes difficult to manufacture the transducer. In addition, the magnitude of electric field increases, so that accuracy and reliability of the transducer and reflectors deteriorates.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known in this country to a person of ordinary skill in the art.